1. Field of the Invention
This invention relates generally to an apparatus for carrying electrical current in petroleum well drilling and logging tools. More specifically, this invention relates to a downhole apparatus for carrying high electrical current between a compartment having relatively high pressure and another compartment having relatively low pressure.
2. Description of the Related Art
Modern petroleum well drilling and logging tools frequently require the passage of electrical current between an area having relatively high pressure and another area having relatively low pressure. For example, in many pulsed nuclear magnetic resonance (NMR) measuring while drilling (MWD) tools, an antenna disposed generally on the periphery of the tool is used both to transmit radio frequency electromagnetic wave pulses into the surrounding earth formation and to receive NMR signals from the formation. In such tools, tuning capacitors are utilized in the antenna electronics (driving circuitry) to match the impedance of the antenna so that the antenna will resonate at the desired natural frequency. However, the tuning capacitors are sensitive items and require protection from the high pressures and temperatures of the harsh borehole environment.
Before the advent of tools such as that described in U.S. Pat. No. 5,557,201, issued to Kleinberg et al. on Sep. 17, 1996, that problem was solved by selecting capacitors with minimal pressure and temperature sensitivities and isolating the capacitors from the borehole fluids in an oil-filled compartment of the drill collar. The compartment seal separated the capacitor compartment from the borehole fluids, but the seal did not form a pressure seal and therefore the compartment realized the ambient borehole pressure. Consequently, the compartment was filled with oil to transmit the ambient pressure uniformly around the capacitors and thereby prevent the capacitors from being crushed by the high differential pressure. Moreover, because oil expands and contracts with changing temperature and pressure, those earlier devices had to include a means of varying the volume of the compartment to compensate for the temperature and pressure changes. Thus, such a scheme was very cumbersome.
Tools such as the '201 apparatus solved that problem by housing the antenna driving circuitry in a compartment that was not only sealed off from the borehole fluids but was also sealed off at constant atmospheric pressure. Thus, the compartment was simply filled with air instead of oil, and there was no need for a volume-regulation device. That method of protecting the capacitors made the manufacturing of the tool much simpler and less costly. However, because the pressure in the vicinity of the antenna (i.e., the borehole environment) is much higher than the pressure in the capacitor compartment, the apparatus for feeding the antenna into the capacitor compartment must withstand a severe pressure differential. For example, it is not uncommon for the borehole ambient pressure to be 1700 to 1900 times higher than standard atmospheric pressure. With such a high pressure differential, one would desire to minimize the area of the antenna feed-through apparatus to minimize the force acting on it. On the other hand, because certain NMR MWD tools require a very high electrical power in the antenna (for example, on the order of 10,000 watts at 600 volts and 16.7 amperes), the area of the feed-through apparatus must be large enough to accommodate a conductor of sufficient size to meet the high power requirement. Additionally, the feed-through area must be large enough to supply a sufficient gap between the two leads of the antenna loop.
Although several existing U.S. patents disclose various designs for carrying electrical current, none of the existing designs appears to be directed to solving the aforementioned problems. For example, U.S. Pat. No. 5,203,723, issued to Ritter on Apr. 20, 1993, discloses a pin-type electrical connector comprising one or more conductor pins disposed through a plastic body for use in high pressure and high temperature downhole environments. The '723 design is primarily directed to providing a hermetically sealed electrical connector between a relatively high pressure area and a relatively low pressure area and to improving connector performance and service life over a large number of elevated temperature and pressure cycles. However, the '723 design does not appear to be directed to providing a conductive path for very high electrical current through as small a cross-sectional area as possible.
Similarly, U.S. Pat. No. 4,237,336, issued to Kostjukov et al. on Dec. 2, 1980, discloses a thermocompensating electrical conductor for providing an electrical path between a dean zone and a contaminated zone, such as a nuclear reactor. The '336 conductor, which is preferably configured in the shape of a wave in the direction of electrical current flow and preferably comprises a stack of crimped metal strips, is primarily directed to improving thermal compensation and reducing electrodynamic loading when used for heavy electrical currents. Again, however, the '336 device does not appear to be directed to providing a conductive path for very high electrical current through as small a cross-sectional area as possible.
U.S. Pat. No. 4,222,029, issued to Marquis et al. on Sep. 9, 1980, discloses a vibration isolator having a sinuously configured, electrically conductive wire disposed within an elastomeric resilient member. Similar to the conductor of the '336 device, the wire of the '029 device has a wave-like shape in the direction of electrical current flow. The wave-like shape of the wire is directed to permitting linear extension of the wire in the direction of electrical current flow without breaking when the device is flexed by vibratory loads. However, the '029 device does not appear to be directed to providing a conductive path for very high electrical current through as small a cross-sectional area as possible, and the '029 device is not directed to accommodating a high pressure differential.
U.S. Pat. No. 3,994,552, issued to Selvin on Nov. 30, 1976, discloses a cylindrical metal electrical connector having a bellows configuration in the axial direction for connecting submersible pipes. The bellows configuration is directed to alleviating axial manufacturing tolerance problems. Once again, however, the '552 device is not directed to solving the need for a downhole electrical connector capable of carrying high electrical currents between a high pressure compartment and a low pressure compartment through as small a cross-sectional area as possible.
It would, therefore, be a significant advancement in the art to provide an improved downhole apparatus for supplying high electrical current between a compartment having relatively high pressure and another compartment having relatively low pressure through as small a cross-sectional area as possible.